Conventionally, an endoscope which includes a long and thin insertion part which is inserted into a cavity or the intra-corporeal of a human being for observation, etc., has been widely used in both the industrial field and the medical field. Furthermore in recent years, there have been a great need to measure the size and the depth of flaws and cracks in the industrial field, and to perform surgery using an endoscope in the medical field. Moreover in the medical field, using stereo images to recognize depth information is well known.
A conventional endoscope in which stereo observation are possible is described in Japanese Laid-open Patent Publication No. 8-29701. As shown in FIG. 6, two objective-lens systems are arranged in parallel and at the end of an endoscope insertion part. The endoscope conducts parallax stereoscopic vision by receiving an image from two image-pick-up devices (henceforth, CCD) and shifting the image due to the positional differences of the two CCDs.
In the optical system with two CCDs, since the number of pixels can be increased as compared to one CCD, it is effective in improving the image quality of stereo images and the precision of measurements. However, by arranging two CCDs in parallel, it then becomes difficult to reduce the size of an endoscope which makes it difficult to observe a narrow site or perform minimum invasive surgery.
A conventional endoscope having an optical system in which an image with a parallax is formed on one CCD is described in Japanese Laid-open Patent Publication No. 7-35989. As shown in FIG. 7, this optical system has one CCD on the extension line of the respective optical axis of the two object optical systems arranged in parallel to form two images. The distance between the two optical axes is hereinafter referred to as “the optic-axial distance”.
On the one hand, in recent years the trend has been toward smaller-sized CCDs. However, if the optimum optic-axial distance is determined such that a part of the object optical system nearest to the object has a moderate parallax, and an image is formed on the CCD with that optic-axial distance, as shown in FIG. 8, the center of an image will be positioned at the end of the CCD, if the CCD is small. That is, the area of the images at the right and left sides of the CCD, marked with diagonal lines in FIG. 8, decreases and causes interference with the stereoscopic vision and measurements.
Conversely, if the size of the CCD to be used is determined in accordance with the size of an endoscope, and the distance between the centers of the optimum images on the CCD is also determined, as shown in FIG. 9, there is a possibility that a parallax required for a measurement or for stereoscopic vision cannot be obtained if the optic-axial distance is equal to the distance between the centers of the images on the CCD for the nearest object to the object optical system.
Therefore, in order to obtain a moderate parallax required for a measurement or a stereoscopic vision as well as an acceptable image on the CCD, the optic-axial distance of an object optical system, and the distance between the centers of the images on CCD need to be varied.
However, the optic-axial distance of an object optical system influences a parallax which is important at the time of a stereoscopic-vision observation and the precision at the time of measurement. If the optic-axial distance of the object optical system nearest to the object is narrow, a parallax will decrease so that depth information becomes hard to obtain when carrying out a stereoscopic vision and a measurement error becomes large when carrying out a measurement operation.
Conversely, if the optic-axial distance of the object optical system nearest to the object is wide, a parallax will become large. Although the precision of a measurement improves, the problem will arise that the end of an endoscope becomes large. And since a parallax is too large when carrying out a stereoscopic-vision observation, it is hard to observe on the contrary.
A parallax depends not only on the optic-axial distance of the object optical system nearest to the object but also on the distance to the object to be observed. In other words, the closer the object is, the larger the parallax becomes, and the farther it is, the smaller the parallax becomes.
Based on the above, in order to obtain a moderate parallax required for measurement and for a stereoscopic-vision observation depending on the observation distance, and in order not to make the size of an endoscope large, the optic-axial distance of the object optical system at a part thereof nearest to the object should be determined.
A conventional example in which the optic-axial distance of an object optical system is different from the distance between the centers of the images on the CCD is described in the Japanese Laid-open Patent Publication No. 62-215221.
In this example, as shown in FIG. 10, by sandwiching a parallel flat dioptric element with a pair of optical systems, the optic-axial distance of an object optical system is narrowed towards the center of the image on the CCD. In such an optical system, if the CCD is small, the parallel flat dioptric element must be small. However, it is difficult to design and manufacture a small parallel flat dioptric element, and still more difficult to sandwich it with a pair of optical system.